Hereinafter, the explanation will be given using steel as a typical example of a metal. Non-metallic inclusion particles existing in the steel include alumina type inclusions formed as the result of the reaction between oxygen in the steel and aluminum added in the case of an aluminum killed steel, slag type inclusions containing lime/silica, etc., and resulting from a steel making slag, powder type inclusions resulting from a casting mold lubricant in continuous casting, and so forth. Since these inclusions result in defects such as flows and breakage in intermediate products, during rolling of thin sheets, wire materials, etc., or in final products, evaluation of these inclusions by various methods have bean carried out in the past for the purpose of quality control.
If any defects are found in the final product, on the other hand, it is a serious problem to discard the product at the final stage from the aspect of the production cost because the product is produced through various production steps. It is therefore desirable to evaluate quality at an early stage of the production. Particularly because the existence of the inclusions is determined at the stage of refining/solidification of the metal, various evaluation technologies have been conducted in the past.
The evaluation technology of the inclusions of the steel among the metals is described, for example, in "Steel Handbook, 3rd Edition", II Pig Iron & Steel Making (edited by Japan Iron & Steel Institute of Japan, published by Maruzen, Oct. 15, 1979). Examples of the evaluation methods include a total oxygen (T[O]) method based on the oxygen concentration in the steel, a slime method by electrolytic extraction used for evaluating large inclusions, a microscopic method for evaluating the inclusions by magnifying and observing the section of a metal, and so forth. Due to their respective features, these technologies are limited by the kind of inclusions as the investigation object and the sizes of the inclusions as tabulated in Table 1, and they are not free from the problem, either, that a long time is necessary depending on the evaluation method.
It is known that information of intermediate products is not sufficient so as to estimate the product defects. In other words, as shown in Table 1, the conventional means involves the problems that the evaluation sample does not sufficiently represent the quality of the intermediate product and a long time is necessary for the evaluation of the sample, and those methods which invite excessively great super-heat during melting such as an EB (electron beam melting) method involve the problem that the inclusions are denatured during evaluation.
The slime method has been widely employed as a method having relatively high accuracy, but an extremely long time of several days to dozens of days is necessary to electrolyze about 1 kg sample as a whole.
When the evaluation is made by a small amount of metal sample, a metal piece sample of a part of large amounts of metal is evaluated. Therefore, to strictly evaluate the cleanliness of the whole metal, a large number of samples must be collected from the same metal piece, and the problem to be solved is to speed up the evaluation of the cleanliness.
TABLE 1 __________________________________________________________________________ particle diameter evaluation quantity name of inclusions & necessary time others __________________________________________________________________________ microscope up to 40 .mu.m 100 positions, 25 mm.sup.2 several days T[O] -- -- slime at least 40 .mu.m several kg, several to dozens of days EB up to 200 .mu.m 2 g (several pcs) one components evaporate day due to reduced pressure This Invention not limited hundred to thousand components do not grams, about 10 min. evaporate due to Ar atmospheric pressure __________________________________________________________________________
On the other hand, though the melting means is different from the EB method, an induction melting extraction method using a cold crucible method is conceivable as the same melting extraction method. In other words, this method eliminates the problems such as high temperature melting of the EB method and the resulting modification of the inclusions, and insufficiency as the representative value by the evaluation volume of the small amount. A method of measuring the inclusions of the surface of the sample produced by this cold crucible levitation-melting method is described, for example, in "Evaluation of Alloy Cleanness in Superclean Materials", K. C. Mills et al., Turkdogan Symposium Proceedings, pp. 105-112 (1994). The method of this reference inspects the surface inclusions by a scanning electron microscope. However, this reference points out only the problem as the evaluation method by the characteristics of the cold crucible itself, but does not teach the method of evaluating the non-metallic inclusions over a wide area of the metal surface industrially, economically and quickly.
FIGS. 1(a) and 1(b) are explanatory views of the principal portions of a cold crucible apparatus, wherein FIG. 1(a) is an explanatory plan view, and FIG. 1(b) is an explanatory view of the longitudinal section taken along A--A of FIG. 1(a). In FIG. 1, reference numerals (1-1, . . . , 1-8) denote eight, for example, copper segments which together form a crucible and the inside of which is cooled with water. They are disposed adjacent to one another with the gap slits 3 interposed at a plurality of substantially equidistant positions and form the crucible. Reference numeral 2 in the drawings denotes an induction coil, which is so disposed as to encompass the crucible.
FIGS. 2(a) and 2(b) are explanatory views of the operation of the cold crucible. When a high frequency current flows through the induction coil 2 in a direction indicated by an arrow 5, an inducted electromotive force occurs in a direction indicated by an arrow 6-1 occurs on the side of the induction coil 2 of the segments 1. Since the segments 1 are spaced apart from one another by the slits 3, however, the induction current does not flow through other adjacent segments, but flows as an induction current in a direction indicated by an arrow 6-2 on the opposite side to the induction coil. Reference numeral 4 in the drawing represents the metal sample. An eddy current flows through the metal sample 4 in a direction indicated by an arrow 7 due to the induction current in a direction indicated by an arrow 6-2. The metal piece 4 is heated by the eddy current in the direction of the arrow 7 and is melted. In this instance, since the eddy current flows through the molten metal 4 in the direction of the arrow 7, repulsion 8 acts in the center direction of the metal due to the induction current in the direction of the arrow 6-2 that flows through the segments 1, and this repulsion 8 keeps the molten metal 4, under a levitating and non-contact state, away from the segments 1.
The cold crucible method melts the metal sample, due to levitation, in a non-oxidizing atmosphere and holds the levitating molten metal. During this retention time, the non-metallic inclusions in the metal sample are discharged to the surface of the molten metal as indicated by reference numeral 9 in FIG. 2(b). When the current to be passed through the coil is cut off after retention for a predetermined time, the molten metal is solidified while the non-metallic inclusions gather on the surface thereof. The cleanliness of the metal piece is evaluated by measuring the non-metallic inclusions gathered on the surface of the solidified body.
According to the prior art method which measures the non-metallic inclusions scattered inside the metal piece, measurement is complicated and requires a long measurement time but according to the cold crucible method, the measurement of the non-metallic inclusions gathering on the surface can be easily made because they gather on the surface of the solidified body and, moreover, within a short time. According to the prior art method which measures the non-metallic inclusions contained and scattered in the metal, the sample is extremely small, and is not correct as a representative value of the steel. On the other hand, because the cold crucible method can levitate several grams to several kilograms of the metal sample, the quantity of the sample is greater than before, and evaluation can be made more correctly over a typical values of the steel.